The present invention relates generally to the processing of titanium aluminides to form articles and reinforced articles. More particularly, it relates to a plasma-spray technology which is applied to modify the composition of titanium aluminides as they are being processed and to the articles which are formed by such processing.
It is known that the titanium aluminides, and particularly intermetallic compositions of titanium and aluminum, have a very desirable set of properties for use at higher temperatures. Intermetallic compositions of titanium and aluminum can be employed at temperatures of 1,000.degree. F. and higher. One of the problems associated with the use of the aluminides is that they tend to be somewhat brittle at room temperatures. However, recent developments have permitted the formation of modified titanium aluminides which have desirable properties at elevated temperatures but which also have significant ductility at room temperatures. One such modified composition is described in U.S. Pat. No. 4,842,819.
The titanium based materials have a desirable set of properties at elevated temperatures which makes it appropriate to consider them for use in high temperature, high stress applications such as in gas turbines, and in jet engines. One of the ways in which it is proposed to process the titanium compositions is through a plasma-spray deposition process. This process involves formation of a plasma by passing a current through a gas to cause ionization of the gas and by then passing fine particles of the material to be spray deposited through the plasma so that the finely divided material is heated to a temperature above its melting point as it passes through the plasma. In this way, deposits of material which are difficult to process have been made. For example, deposits of refractory metals and ceramic materials have been made in this fashion. The subject invention concerns the plasma-spray deposit of a titanium aluminide to form either a monolithic deposit or to form a composite of titanium aluminide with a reinforcing structure. The formation of such structures is disclosed in U.S. Pat. Nos. 4,775,547; 4,782,884; 4,786,566; 4,805,294; 4,805,833; and 4,838,337; assigned to the same assignee as the subject invention.
I have found that the mechanical properties of titanium alloys are sensitive to the composition of the powder used in this spray process. In the manufacture of titanium alloy powders, it is not uncommon for the powder composition to deviate from the desired composition. If the deviation of the composition from a desired standard is large, the deposit made from the powder will be defective because of the inappropriate proportions of ingredients and the alloy will have to be discarded or will have to be remelted. Such discarding or remelting of finished plasma-spray deposited pieces is very expensive and considerable financial cost to the user can result.
The formation of powder with a very precise ratio of titanium to aluminum is also not a simple or easy step or process and it is not uncommon for powder compositions which are prepared to be somewhat out of specification in that the proportions of the ingredient aluminum and titanium do not match those of the specification. Such "out-of-spec" powders can be very costly to the powder manufacturer and result in a need for very expensive reprocessing.
Further, because of the very strong tendency of titanium based materials to absorb and react with oxygen, the reprocessing of titanium-based materials, particularly the reprocessing of powder, can result in the material having a higher than desired oxygen content and can result in the material being out of specification for this reason.
I have discovered that it is possible to modify powder compositions in subsequent processing steps and in this way to avoid or overcome some of the inherent difficulties in the powder processing of titanium-based materials.